Dynamically creating physics objects in WPF

I'm trying to make a behaviour that's effectively a particle emitter, but I can only use it to create rectangles. Unfortunately I kept failing and ended up trying to match the silverlight examples (Ragdoll example where you click the button to create more
dolls).

In my project, I have a UserControl named Rock, containing a canvas with a path that looks like a rock (and the path has a physicsObjectBehavior attached.) The following code is in MainWindow.xaml.cs triggered by a timer.

What if you place the Path into a Canvas (in Blend right click the Path and select Group Into/Canvas), and then apply the PhysicsObjectBehavior (and other physics behaviors) to the Canvas instead of the Path?

There are sometimes issues with nested objects but I think they are worked around by grouping things into a Canvas and then using the Canvas to apply Physics Behaviors.